For factories or facilities considering connecting field devices (such as instruments and actuators) to control and monitoring systems via wireless networks, there are typically two main wireless protocols to choose from: ANSI/ISA100.11a and WirelessHART. Both are adopted as global standards by the International Electrotechnical Commission (IEC) (IEC 62734 - Wireless Communication Networks and Communication Profiles, and IEC 62591 - Industrial Networks: Wireless Communication Networks and Communication Profiles, respectively) and have been in use for approximately 10 years.
These two approaches share many similarities. For example, they use the same low-power IEEE 802.15.4 radio. WirelessHART's mesh network is a self-healing and self-organizing network. ISA100.11a is also self-healing, with each node capable of having redundant paths, which the user specifies for managing the network. However, the way the networks are organized differs significantly. It largely depends on how the network forms communication paths and uses mesh technology, which determines how individual field devices communicate bidirectionally with each other.
Figure 1: Routers deployed between various instruments and gateways help manage data traffic and minimize reliance on the grid. Image credit: Yokogawa Electric Corporation.
WirelessHART self-organization
Both protocols originated in the early 21st century, a time when automation technology in process manufacturing was undergoing tremendous changes. The fieldbus wars of those early years have faded away, but they left a deep impression on many automation vendors: if something is too complex to operate, even a technological advantage may reduce the likelihood of commercial application.
WirelessHART emerged on the market with its simple and quick configuration, perhaps hoping to avoid the criticisms similar to those surrounding Foundation Fieldbus and PROFIBUS PA buses: they are too complex for general factory personnel to implement easily. The WirelessHART protocol uses many of the tools and technologies of the traditional wired instrument HART protocol, but communicates wirelessly.
WirelessHART possesses self-organizing capabilities, allowing devices on a network to automatically determine how to communicate with each other to exchange data. A single instrument's transmitter can send data to neighboring devices, which then pick up the data and pass it on to each other until it reaches the gateway. This introduces some latency, but this is usually a minor consideration. The network's self-organizing capability is dynamic, adjusting in real-time to changing conditions. While this technology is effective and has its advantages, it also presents challenges:
WirelessHART's self-management eliminates the need for most external management tools. The network creates its own communication paths, but manual bypassing is not possible.
WirelessHART's self-organizing nature means scalability can be an issue. Any gateway has a limit to the number of devices it can handle, for example, a maximum of 100. Self-organization doesn't always mean self-optimization. It can find a communication path with sufficient bandwidth for a given device, but that path isn't necessarily the most direct. Network designers can use diagnostic tools to see how devices communicate, but WirelessHART cannot indicate which devices are communicating with each other. If the path used is less than ideal, the mechanism for creating new paths requires placing other devices in the network to allow for better paths. Adding more devices doesn't necessarily eliminate pinch points or reduce the number of hops required for a signal to reach the gateway.
Networks adapt their capabilities as needed, providing an attack surface for network intruders to attempt and explore. For example, a "wormhole" attack targets a network specifically designed for a particular purpose; even if the attacker doesn't compromise any hosts or break any encryption, it can still disrupt normal communication paths. With defensive technologies, successfully launching an attack is not easy, but networks relying on mesh communication have this fatal weakness.
The physical layout of a network can lead to communication paths that rely on a small number of strategically placed nodes to transmit data from a large number of devices. These pinch points can place a heavy burden on these strategic nodes, and if lost due to battery failure or some other disruption to the path, they could cut off a major part of the network.
The WirelessHART protocol acknowledges the possibility of these pinch points and their potential impact: First, wireless devices that must communicate through pinch points may experience reduced communication reliability. Second, the bandwidth of wireless devices that must communicate through pinch points may be limited, and network performance may be adversely affected. Third, wireless devices acting as pinch points will consume additional power to transmit the increased message load. This is particularly important for battery-powered devices (leading to reduced battery life) or devices that rely on clean energy (e.g., solar-powered devices).
Many factors can cause network slippage. For example, slippage can be caused by improper network design or installation, constantly changing radio frequency (RF) environment, changes in the network physical space (affecting the RF environment), and wireless devices ceasing service.
The WirelessHART network analysis tool can monitor communication paths and the status of member devices, such as battery status. The software can identify locations where grips have formed and alert operators to the presence of grips. Unfortunately, the network cannot take any action to correct this condition because the solution always involves adding or moving equipment to establish a more favorable communication path. Someone must rearrange things until the network finds its own solution or adds a new gateway in another location; the setup may involve segmenting the network.
Managed and Self-organized
The ISA100 standards committee was established to prepare a series of wireless communication standards for industrial automation applications. ISA100 Working Group 3 was responsible for the development of ISA100.11a, and products have been shipped under the ISA100 Wireless brand since 2013.
Larger-scale standardization efforts began with networks supporting complex manufacturing environments, extending beyond just field devices and instruments. The standard's creators also believed that achieving maximum performance and security should outweigh oversimplification. To meet the performance and control requirements of critical users, some network planning and management are necessary. However, this must be done without compromising fieldbus availability.
The ISA100.11a wireless instrument network can be configured as a self-organizing mesh network, just like WirelessHART, but it is not the only option. Many more tools and techniques are available, allowing users to choose the best approach for a given application and plant environment.
Simple planning and consideration during the design phase of implementing the ISA100.11a project will significantly improve all the radio links upon which the network depends. Understanding basic signal propagation can guide the placement of equipment and antennas, avoiding the influence of steel tanks and steel structures ubiquitous in complex factories, and avoiding the drawbacks of mesh networks.
ISA100.11a can use a router as a relay point (as shown in Figure 1) instead of directly connecting all devices in a large group of devices to the same gateway. These devices collect data from various wireless instruments and then transmit the data to the gateway. The router is simply a wireless transmitter, such as a temperature transmitter, that can be configured to actively communicate with the gateway.
This avoids sending signals between multiple field devices, which would reduce data movement speed and increase power consumption for each device. This approach to implementing ISA100.11a uses mesh networking only when needed as a means of resolving network interruptions, rather than keeping it constantly active for communication between each device.
Because they do not require continuous communication, field instruments with very low refresh rates (such as level indicators on large water tanks) can remain dormant for extended periods. This saves power as they do not need to be constantly active and communicate with other devices. If a major network outage disrupts the link between the field instrument and its primary router, the device will automatically contact the secondary router.
Effective Implementation of Wireless Networks
One of the most effective practices is to place the router in a high position so that the gateway has a clear "line of sight" and can look down at all the devices (Figure 2). While most wireless field instruments have integrated antennas, if the router does not have a clear "line of sight," the antenna may have to be moved to avoid obstructions. Additionally, if a single instrument is attempting to send a signal to a specific point, a directional antenna can be used to increase signal strength.
Figure 2: Effective router deployment can make communication between the two ends of the network more reliable.
Engineers and technicians using network management tools are responsible for establishing these communication links. Once established, these links typically remain static because the equipment itself is located in a specific area. Interruptions may occur, such as communication congestion disrupting the radio link, but the network generally does not require constant adjustments. A well-deployed ISA100.11a network using these methods can maintain stable operation for many years. Some vulnerable devices may require antenna relocation, but this is not difficult to implement.
The ISA100 series wireless standard is based on a concept suitable for process plant environments. From its inception, it has been used in refineries, chemical plants, and other challenging conditions. It also encompasses many forms of wireless communication beyond instrumentation.
On the other hand, WirelessHART chose to reduce development effort by adopting and improving existing technologies. While avoiding “starting from scratch” is generally a wise approach, in this case, technologies designed for a specific purpose, intended to be used to build a network under changing conditions, can present certain challenges.
Choosing which wireless network to implement in a facility is crucial, and for a given operating condition, the optimal choice depends on a number of factors.
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